Polarizing optical system

ABSTRACT

There is provided an optical system, including a light-transmitting substrate having at least two major surfaces parallel to each other edges, and an optical device for coupling light into the substrate by total internal reflection. The device includes a polarization sensitive reflecting surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 17/506,841filed 21 Oct. 2021 for POLARIZING OPTICAL SYSTEM, which is acontinuation of application Ser. No. 16/823,454 filed Mar. 19, 2020 forPOLARIZING OPTICAL SYSTEM, which is a continuation of application Ser.No. 16/013,983 filed 21 Jun. 2018 for POLARIZING OPTICAL SYSTEM, nowU.S. Pat. No. 10,598,937, granted Mar. 24, 2020, which is a continuationof application Ser. No. 15/289,774 filed Oct. 10, 2016 for POLARIZINGOPTICAL SYSTEM, now U.S. Pat. No. 10,048,499, granted Aug. 14, 2018 mwhich is a divisional of application Ser. No. 12/092,818 filed May 6,2008 for POLARIZING OPTICAL SYSTEM, now U.S. Pat. No. 9,551,880, grantedJan. 24, 2017.

FIELD OF THE INVENTION

The present invention relates to substrate-guided optical devices, andmore particularly, to devices which include a plurality of reflectingsurfaces carried by a common light-transmissive substrate, also referredto as a light-guide.

The invention can be implemented to advantage in a large number ofimaging applications, such as, for example, head-mounted and head-updisplays, cellular phones, compact displays, 3-D displays, compact beamexpanders as well as non-imaging applications such as flat-panelindicators, compact illuminators and scanners.

DESCRIPTION OF RELATED ART

One of the important applications for compact optical elements is inhead-mounted displays, wherein an optical module serves both as animaging lens and a combiner, in which a two-dimensional display isimaged to infinity and reflected into the eye of an observer. Thedisplay can be obtained directly from either a spatial light modulator(SLM), such as a cathode ray tube (CRT), a liquid crystal display (LCD),an organic light emitting diode array (OLED), or a scanning source andsimilar devices, or indirectly, by means of a relay lens or an opticalfiber bundle. The display comprises an array of elements (pixels) imagedto infinity by a collimating lens and transmitted into the eye of theviewer by means of a reflecting or partially reflecting surface actingas a combiner for non-see-through and see-through applications,respectively. Typically, a conventional, free-space optical module isused for these purposes. As the desired field-of-view (FOV) of thesystem increases, such a conventional optical module becomes larger,heavier, bulkier and therefore, even for a moderate performance device,impractical. This is a major drawback for all kinds of displays, butespecially in head-mounted applications, where the system mustnecessarily be as light and as compact, as possible.

The strive for compactness has led to several different complex opticalsolutions, all of which, on the one hand, are still not sufficientlycompact for most practical applications, and, on the other hand, suffermajor drawbacks in terms of manufacturability. Furthermore, theeye-motion-box (EMB) of the optical viewing angles resulting from thesedesigns is usually very small—typically less than 8 mm. Hence, theperformance of the optical system is very sensitive, even to smallmovements of the optical system relative to the eye of the viewer, anddoes not allow sufficient pupil motion for convenient reading of textfrom such displays.

SUMMARY OF THE INVENTION

The present invention facilitates the design and fabrication of verycompact light-guide optical elements (LOE) for, amongst otherapplications, head-mounted displays. The invention allows relativelywide POVs together with relatively large EMB values. The resultingoptical system offers a large, high-quality image, which alsoaccommodates large movements of the eye. The optical system offered bythe present invention is particularly advantageous because it issubstantially more compact than state-of-the-art implementations and yetit can be readily incorporated even into optical systems havingspecialized configurations.

The invention also enables the construction of improved head-up displays(HUDs). Since the inception of such displays more than three decadesago, there has been significant progress in the field. Indeed, HUDs havebecome popular and they now play an important role, not only in mostmodern combat aircraft, but also in civilian aircraft, in which HUDsystems have become a key component for low visibility landingoperation. Furthermore, there have recently been numerous proposals anddesigns for HUDs in automotive applications where they can potentiallyassist the driver in driving and navigation tasks. Nevertheless,state-of-the-art I-IUDs suffer several significant drawbacks. All HUDsof the current designs require a display source that must be offset asignificant distance from the combiner to ensure that the sourceilluminates the entire combiner surface. As a result, thecombiner-projector HUD system is necessarily bulky and large, andrequires a considerable installation space, which makes it inconvenientfor installation and at times even unsafe to use. The large opticalaperture of conventional I-IUDs also poses a significant optical designchallenge, rendering the HUDs with either compromised performance, orleading to high cost wherever high-performance is required. Thechromatic dispersion of high-quality holographic HUDs is of particularconcern.

An important application of the present invention relates to itsimplementation in a compact HUD, which alleviates the aforementioneddrawbacks. In the HUD design of the current invention, the combiner isilluminated with a compact display source that can be attached to thesubstrate. Hence, the overall system is very compact and can readily beinstalled in a variety of configurations for a wide range ofapplications. In addition, the chromatic dispersion of the display isnegligible and, as such, can operate with wide spectral sources,including a conventional white-light source. In addition, the presentinvention expands the image so that the active area of the combiner canbe much larger than the area that is actually illuminated by the lightsource.

A further application of the present invention is to provide a compactdisplay with a wide FOV for mobile, hand-held application such ascellular phones. In today's wireless internet-access market, sufficientbandwidth is available for full video transmission. The limiting factorremains the quality of the display within the device of the end-user.The mobility requirement restricts the physical size of the displays,and the result is a direct-display with a poor image viewing quality.The present invention enables a physically very compact display with avery large virtual image. This is a key feature in mobilecommunications, and especially for mobile internet access, solving oneof the main limitations for its practical implementation. The presentinvention thereby enables the viewing of the digital content of a fullformat internet page within a small, hand-held device, such as acellular phone.

The broad object of the present invention, therefore, is to alleviatethe drawbacks of state-of-the-art compact optical display devices and toprovide other optical components and systems having improvedperformance, according to specific requirements.

The invention therefore provides an optical system, comprising alight-transmitting substrate having at least two major surfaces parallelto each other and edges, and an optical device for coupling light intosaid substrate by total internal reflection, characterized in that saiddevice for coupling light includes a polarization sensitive reflectingsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

Many of the attendant advantages of this invention will be readilyappreciated as the same becomes better understood by reference to thefollowing detailed description considered in conjunction with theaccompanying drawings in which like reference numeral designate likeparts throughout the figures thereof and wherein:

FIG. 1 is a side view of a generic form of a prior art folding opticaldevice;

FIG. 2 is a side view of an exemplary light-guide optical element;

FIGS. 3A and 3B illustrate the desired reflectance and transmittancecharacteristics of selectively reflecting surfaces for two ranges ofincident angles;

FIG. 4 is a schematic sectional-view of a reflective surface embeddedinside a light-guide optical element;

FIG. 5 illustrates an exemplary embodiment of a light-guide opticalelement embedded in a standard eyeglasses frame;

FIG. 6 illustrates an exemplary embodiment of a light-guide opticalelement embedded in a hand carried display system;

FIGS. 7A to 7D are diagrams illustrating a method for fabricating anarray of partially reflecting surfaces along with a coupling-inreflecting surface;

FIG. 8 is a side view of another exemplary light-guide optical element;

FIG. 9A to FIGS. 9D, 10A to 10D, 11A and 11B are diagrams illustratingother methods for fabricating an array of partially reflecting surfacesalong with a coupling-in reflecting surface;

FIG. 12 is a diagram illustrating a system for coupling-in polarizedinput waves into a light-guide optical element in accordance with thepresent invention;

FIG. 13 is a side view of an exemplary light-guide optical element inaccordance with the present invention;

FIG. 14 illustrates two marginal rays coupled into a light-guide opticalelement by a coupling-in conventional reflecting surface;

FIG. 15 illustrates two marginal rays coupled into a light-guide opticalelement by a coupling-in polarization-sensitive reflecting surface, inaccordance with the present invention;

FIG. 16 is a diagram illustrating another embodiment for coupling-ininput waves into a light-guide optical element, exploiting a collimatinglens, in accordance with the present invention;

FIG. 17 illustrates two marginal rays coupled into a light-guide opticalelement by a coupling-in polarization-sensitive reflecting surfaceutilizing a collimating lens, in accordance with the present invention;

FIG. 18 is a diagram illustrating a device for collimating andcoupling-in input waves from a display source into a light-guide opticalelement, in accordance with the present invention;

FIG. 19 is a diagram illustrating another embodiment for collimating andcoupling-in input waves from a display source into a light-guide opticalelement, in accordance with the present invention;

FIG. 20 is a diagram illustrating yet another embodiment for collimatingand coupling-in input waves from a display source into a light-guideoptical element, in accordance with the present invention; and

FIG. 21 is a diagram illustrating still a further embodiment forcoupling-in unpolarized input waves into a light-guide optical element,in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a conventional folding optics arrangement, whereinthe substrate 2 is illuminated by a display source 4. The display iscollimated by a collimating lens 6. The light from the display source 4is coupled into substrate 2 by a first reflecting surface 8 in such away that the main ray 10 is parallel to the substrate plane. A secondreflecting surface 12 couples the light out of the substrate and intothe eye of a viewer 14. Despite the compactness of this configuration,it suffers significant drawbacks; in particular, only a very limited POVcan be affected. As shown in FIG. 1 , the maximum allowed off-axis angleinside the substrate is:

.alpha.max=arctan (T−d eye 21),  (1) ##EQU00001##

wherein T is the substrate thickness;

d.sub.eye is the desired exit-pupil diameter, and

1 is the distance between reflecting surfaces 8 and 12.

With angles higher than .alpha..sub.max the rays are reflected from thesubstrate surface before arriving at the reflecting surface 12. Hence,the reflecting surface 12 will be illuminated at an undesired directionand ghost images appear.

Therefore, the maximum achievable FOV with this configuration is:

FOV.sub.max.apprxeq.2.nu..alpha..sub.max,  (2)

wherein v is the refractive index of the substrate. Typically therefractive index values lie in the range of 1.5-1.6.

Commonly, the diameter of the eye pupil is 2 to 6 mm. To accommodatemovement or misalignment of the display, a larger exit-pupil diameter isnecessary. Taking the minimum desirable value at approximately 8 to 10mm, the distance between the optical axis of the eye and the side of thehead, 1, is, typically, between 40 and 80 mm. Consequently, even for asmall POV of 8.degree., the desired substrate thickness would be of theorder of 12 mm.

Methods have been proposed to overcome the above problem, includingutilizing a magnifying telescope inside the substrate and non-parallelcoupling directions. Even with these solutions, however, and even ifonly one reflecting surface is considered, the system's thicknessremains limited by a similar value. The FOV is limited by the diameterof the projection of the reflective surface 12 on the substrate plane.Mathematically, the maximum achievable FOV, due to this limitation, isexpressed as:

FOV max.apprxeq. T tan.alpha.sur−d eye R eye,  (3) ##EQU00002##

wherein .alpha..sub.sur is the angle between the reflecting surface andthe normal to the substrate plane, and

R.sub.eye is the distance between the eye of the viewer and thesubstrate (typically, about 30-40 mm).

Practically tan.alpha..sub.sur cannot be much larger than 1; hence, forthe same parameters described above for a FOV of 8.degree., the requiredsubstrate thickness here is in the order of 7 mm, which is animprovement on the previous limit. Nevertheless, as the desired FOV isincreased, the substrate thickness increases rapidly. For instance, fordesired FOVs of 15.degree. and 30.degree. the substrate limitingthickness is 18 mm or 25 mm, respectively.

To alleviate the above limitations, it is possible to utilize an arrayof at least one parallel selectively reflecting surface, fabricatedwithin a LOE comprising a flat light-transmitting substrate having atleast two major surfaces and edges. FIG. 2 illustrates a sectional viewof an LOE. The first reflecting surface 16 is illuminated by acollimated light waves 18 emanating from a display source (not shown)located behind the device. The reflecting surface 16 reflects theincident light from the source such that the light is trapped inside aplanar substrate 20 by total internal reflection. After severalreflections from the surfaces of the substrate, the trapped waves reachan array of selectively reflecting surfaces 22, which couple the lightout of the substrate into the eye of a viewer 24. Assuming that thecentral wave of the source is coupled out of the substrate 20 in adirection normal to the substrate surface 26 and the off-axis angle ofthe coupled wave inside the substrate 20 is .alpha..sub.in, then theangle .alpha..sub.sur2 between the reflecting surfaces and the substrateplane is:

.alpha.sur 2=.alpha.in 2,  (4) ##EQU00003##

As can be seen in FIG. 2 , the trapped rays arrive at the reflectingsurfaces from two distinct directions 28, 30. In this particularembodiment, the trapped rays arrive at the reflecting surface from oneof these directions 28 after an even number of reflections from thesubstrate surfaces 26, wherein the incident angle .beta..sub.ref betweenthe trapped ray and the normal to the reflecting surface is:

.beta.ref=.alpha.in-.alpha.sur 2=.alpha.in 2,  (5) ##EQU00004##

The trapped rays arrive at the reflecting surface from the seconddirection 30 after an odd number of reflections from the lower substratesurfaces 26, where the off-axis angle is.alpha.′.sub.in=180.degree.-.alpha..sub.in and the incident anglebetween the trapped ray and the normal to the reflecting surface is:

.beta.ref′=.alpha.in′-.alpha. sur 2=180.degree.-.alpha.in-.alpha.sur2=180.degree.-3.alpha.in 2,  (6) ##EQU00005##

In order to prevent undesired reflections and ghost images, it isimportant that the reflectance be negligible for one of these twodirections. The desired discrimination between the two incidentdirections can be achieved if one angle is significantly smaller thanthe other one. It is possible to design a coating with very lowreflectance at high incident angles and a high reflectance for lowincident angles. This property can be exploited to prevent undesiredreflections and ghost images by eliminating the reflectance in one ofthe two directions. For example choosing .beta.′.sub.ref=25.degree. fromEquations (5) and (6) it can be calculated that:

.beta.′.sub.ref=105.degree.;.alpha..sub.in=50.degree.;.alpha.′.sub.in=13-0.degree.;.alpha..sub.sur2=25.degree..  (7)

Now, if a reflecting surface is determined for which .beta.′.sub.ref notreflected but .beta..sub.ref is, the desired condition is achieved.FIGS. 3A and 3B illustrate the desired reflectance behavior ofselectively reflecting surfaces. While the ray 32 (FIG. 3A), having anoff-axis angle of .beta..sub.ref.about.25.degree., is partiallyreflected and coupled out of the substrate 34, the ray 36 (FIG. 3B),which arrives at an off-axis angle of .beta.′.sub.ref.about.75.degree.to the reflecting surface (which is equivalent to.beta..sub.ref.about.105.degree.), is transmitted through the reflectingsurface 34, without any notable reflection.

Hence, as long as it can be ensured that .beta.′.sub.ref, where very lowreflections are desired, will have negligible reflection, similar tothat at .beta.′.sub.ref.about.75.degree., over its angular spectrum,while .beta..sub.ref, will have higher reflections, over its angularspectrum, for a given POV, the reflection of only one substrate modeinto the eye of the viewer and a ghost-free image, can be ensured.

It is important not only to couple the image out of the substratewithout any distortion or ghost image, but also to couple the lightproperly into the substrate. FIG. 4 , which illustrates one method forcoupling-in, presents a sectional view of the reflective surface 16,embedded inside the substrate 20 and couples light 38 a, 38 b from adisplay source (not shown) and traps it inside the substrate 20 by totalinternal reflection. To avoid an image with gaps or stripes, it isessential that the trapped light will cover the entire area of the LOEmajor surfaces. To ensure this, the points on the boundary line 41between the edge of the reflective surface 16 and the upper surface 40of the substrate 20, should be illuminated for a single wave by twodifferent rays that enter the substrate from two different locations: aray 38 a that illuminates the boundary line 41 directly, and another ray38 b, which is first reflected by the reflecting surface 16 and then bythe lower surface 42 of the substrate, before illuminating the boundaryline.

The embodiment described above with regard to FIG. 4 is an example of amethod for coupling input waves into the substrate. Input waves could,however, also be coupled into the substrate by other optical means,including (but not limited to) folding prisms, fiber optic bundles,diffraction gratings, and other solutions.

FIG. 5 illustrates an embodiment that utilizes the coupling-in devicedescribed in FIG. 4 , in which the LOE 20 is embedded in an eyeglassesframe 48. The display source 4, the collimating lens 6, and the foldinglens 50 are assembled inside the arm portions 52 of the eyeglassesframe, next to the edge of the LOE 20. For cases where the displaysource is an electronic element, such as a small CRT, LCD or OLED, thedriving electronics 54 for the display source can be assembled insidethe back portion of the arm 48. A power supply and data interface 56 canbe connected to arm 48 by a lead 58 or other communication means,including radio or optical transmission. Alternatively, a battery andminiature data link electronics can be integrated into the eyeglassesframe.

FIG. 6 illustrates another application that utilizes the coupling-inembodiment described in FIG. 4 . This application is a hand-held display(HHD), which resolves the previously opposing requirements of achievingsmall mobile devices, and the desire to view digital content on a fullformat display, by projecting high quality images directly into the eyeof the user. An optical module including the display source 4, thefolding and collimating optics 6 and the substrate 20 is integrated intothe body of a cellular phone 60, where the substrate 20 replaces theexisting protective cover window of the phone. Specifically, the volumeof the support components, including source 4 and optics 6 issufficiently small to fit inside the acceptable volume for modemcellular devices. In order to view the full screen transmitted by thedevice, the user positions the window in front of his eye 24, observingthe image with high FOV, a large EMB and a comfortable eye-relief. It isalso possible to view the entire FOV at a larger eye-relief by tiltingthe device to display different portions of the image. Furthermore,since the optical module can operate in see-through configuration, adual operation of the device is possible. That is, there is an option tomaintain the conventional cellular display 62 intact. In this manner,the standard, low-resolution display can be viewed through the LOE 20when the display source 4 is shut-off. In a second, virtual-mode,designated for e-mail reading, internet surfing, or video operation, theconventional display 62 is shut-off, while the display source 6 projectsthe required wide FOV image into the eye of the viewer through the LOE20. The embodiment described in FIG. 6 is only an example, illustratingthat applications other than head-mounted displays can be materialized.Other possible hand-carried arrangements include palm computers, smalldisplays embedded into wristwatches, a pocket-carried display having thesize and weight reminiscent of a credit card, and many more.

As illustrated in FIGS. 5 and 6 , there is one major difference betweenthe two applications. In the eyeglasses configuration illustrated inFIG. 5 , the input waves and the image waves are located on the sameside of the substrate, while in the hand-held configuration illustratedin FIG. 6 , the input and the image waves are located on opposite sidesof the substrate. This difference not only affects the shape and size ofthe overall opto-mechanical module, but also determines the internalstructure of the LOE. As illustrated in FIG. 2 , wherein the input wavesand the image waves are located on the same side of the substrate, thecoupling-in element 16 is embedded inside the LOE 20 in a differentorientation to that of the couple-out elements 22. As illustrated inFIGS. 7A to 7D, however, wherein the input and the image waves arelocated on opposite sides of the substrate, the coupling-in element 16is embedded inside the LOE 20 in a similar orientation to that of thecouple-out elements 22. In fact, the reflecting surface 16 is usuallyparallel to the partially reflecting surfaces 22. This difference is notonly cosmetic, but also can significantly influence the fabricationprocedure of the LOE.

It is important that the fabrication process of the LOE will be assimple and inexpensive as possible. Although this is true for all thepotential applications, it is especially critical for applicationswherein the price of the final product must be appropriate for theconsumer market. FIGS. 7A to 7D illustrate a method of fabricating anLOE with the internal structure illustrated in FIG. 8 . First, as seenin FIG. 7A, a group of parallel plates 64 and an associated group ofpartially reflecting surfaces (coated onto these plates) aremanufactured, to the required dimensions. The plates 64 can befabricated from silicate-based materials such as BK-7 with theconventional techniques of grinding and polishing, or alternatively,they can be made from polymer or sol-gel materials usinginjection-molding or casting techniques. Next, a blank plate 66, thecoated plates 64, and a plate having a reflecting surface 68 arecemented together to create a stacked form 70, as illustrated in FIG.7B. A segment 72 (see FIG. 7C) is then sliced off the stacked form bycutting, grinding and polishing, to create the desired LOE 20, shown inFIG. 7D. The procedure illustrated in FIGS. 7A to 7D of coating,cementing, slicing, grinding and polishing can be totally automated todevise a straightforward and inexpensive procedure, which would beappropriate for mass production processes.

For LOEs having the internal structures of FIG. 2 , the fabricationprocedure is much more complicated. FIGS. 9A to 9D illustrate a methodof fabricating an LOE having the required internal structure. The groupof parallel coated plates 64, FIG. 9A, are manufactured as before,however, since the reflecting surface 16 (FIG. 2 ) is no longer parallelto surfaces 22, the plate with the reflecting surface 68 cannot becemented to the stack 70 as before. Therefore, the coupling-out portionof the LOE only can be fabricated in the above manner, that is, only theblank plate 66 and the coated plates 64 are cemented together to createthe stacked form 74, shown in FIG. 9B. A segment 76 (FIG. 9C) is thensliced off the stacked form by cutting, grinding and polishing, tocreate the coupling-out portion 78 (FIG. 9D) of the desired LOE 20.

FIGS. 10A to 10D illustrate how the coupling-in portion 82 of the LOE isprepared separately, in the same manner, as follows: another blank plate79 (FIG. 10A) and the plate 68 having the required reflecting surfaceare cemented together to create a stacked form 80 (FIG. 10B). A segment81, shown in FIG. 10C, is then sliced off the stacked form by cutting,grinding and polishing, to devise the desired coupling-in portion 82(FIG. 10D).

FIGS. 11A and 11B illustrate how the final fabrication step of the LOEis completed. The coupling-out portion 78 and the coupling-in portion 82are cemented together along the common surface 84 to create the finalLOE 20. Since, for most applications, the quality of the opticalsurfaces is critical, the final step of polishing the outer surfaces 26and 27, shown advantageously to be added to the process.

There are some disadvantages to the fabrication process illustrated inFIGS. 9A to 9D, 10A to 10D, 11A and 11B as compared to the processillustrated in FIGS. 7A to 7D. Not only that the number of thefabricating steps is increased from one to three, but mostsignificantly, the last step is particularly complicated and requiresspecial manufacturing attention. The common surface 84 should befabricated, with high accuracy, normal to the major surfaces 26 and 27in both portions 78 and 82. Moreover, the cemented surface 84 might bebroken during the final grinding and polishing step, especially forfabrication of very thin substrates.

Hence, an LOE having an internal structure as illustrated in FIG. 8 , ispreferred over that of FIG. 2 . It is therefore important to find amethod to fabricate an LOE having the former configuration even foroptical systems wherein the input waves and the image waves are locatedon the same side of the substrate. A method which achieves these twoseemingly contradictory requirements and which exploits the fact that inmost micro-display sources, such as LCD or LCOS, the light is linearlypolarized, as illustrated in FIG. 12 . The main difference between theembodiment illustrated here and the embodiment illustrated in FIGS. 2and 8 is that instead of utilizing a uniformly reflecting mirror 16 asthe coupling-in element, a polarizing beamsplitter 86 is embedded insidethe LOE. That is, surface 86 transmits p-polarized and reflectss-polarized light. As will be described, p-polarized and s-polarizedcoupled-in light waves correspond to first and second parts (19 a, 19 b)of coupled-in light waves. In some embodiments, p-polarized coupled-inlight waves correspond to the first part 19 a of coupled-in light wavesand s-polarized coupled-in light waves correspond to the second part 19b of coupled-in light waves. In other embodiments, s-polarizedcoupled-in light waves correspond to the first part 19 a of coupled-inlight waves and p-polarized coupled-in light waves correspond to thesecond part 19 b of coupled-in light waves. As illustrated in FIG. 12 ,the input beam 18 from the collimated display source (not shown) isp-polarized, and therefore is transmitted through surfaces 86. Afterexiting the LOE through the upper surface 27, the light beam impinges ona quarter wave retardation member, e.g., a retardation plate 88 whichconverts the incoming beam into circular polarized light. Thetransmitted beam is then reflected back through the quarter-waveretardation plate 88 by a flat reflecting mirror 90. The reflected beam92, now s-polarized, enters the LOE through the upper surface 27 and isreflected by the polarizing beamsplitter 86. The reflected rays 94 aretrapped inside the LOE by total internal reflection. Apparently, theretardation plate 88 and the reflecting surface 90 could be cementedtogether to form a single element. Alternatively, other methods could beused to combine these into a single element, such as coating areflecting surface on the back side of the retardation plate 88 orlaminating a quarter-wavelength film on the front surface of thereflecting surface 90.

FIG. 13 illustrates the entire structure of the LOE with the coupling-inmechanism described in FIG. 2 . This LOE fulfils the two seeminglycontradicting requirements: The input waves and the image waves arelocated on the same side of the substrate and the coupling-in reflectingsurface is oriented parallel to the partially reflecting coupling-outelements 22. Hence, this structure could be implemented in eyeglassesconfigurations and still be fabricated using the comparatively simpleprocedure illustrated above with reference to FIGS. 7A to 7D.

There are some issues that must be considered when using the coupling-inmethod described herein. One issue is the actual realization of therequired polarizer beamsplitter 86. One method to realize this is byexploiting polarization sensitivity of thin film coatings. The maindrawback of this method is that, as explained above in reference to FIG.3 , the angle .alpha..sub.sur2 between the reflecting surfaces and theincoming waves 18 is in the order of 25.degree.. For these angles, thediscrimination between the S- and the P-polarizations cannot beprominent enough and suitable separation of the two polarizations is notpossible. An alternative solution is presently described, exploitinganisotropic reflecting surfaces, that is, optical surfaces having amajor axis parallel to the surface plane wherein the reflection andtransmission properties of the surface depend strongly in theorientation of the polarization of the incident light in relation to themajor axis of the surface.

A possible candidate for the required anisotropic partially reflectingelement can be a wire grid polarizer, generally in the form of an arrayof thin parallel conductors supported by a transparent substrate. Thekey factor that determines the performance of a wire grid polarizer isthe relationship between the center-to-center spacing or period of theparallel grid elements, and the wavelength of the incident radiation.When the grid spacing or period is much shorter than the wavelength, thegrid functions as a polarizer that reflects electromagnetic radiationpolarized parallel to the grid elements, and transmits radiation of theorthogonal polarization. In this case, the major axis of a wire gridpolarizer is defined as parallel to the array of conductors. Usually, inorder to obtain the best transmission and contrast, the wire gridpolarizer beamsplitter should be used to transmit the p-polarization andreflect the s-polarization, as illustrated in FIG. 12 . It is possible,however, to use the beamsplitter 86 also in the orthogonal orientation,e.g., the major axis of the polarizer is oriented parallel to thepropagation direction of the incident beam. Since the major axis of thepolarizer is now parallel to the electric field of the p-polarizedlight, the polarizer reflects the component of the p-polarized lightwith its electrical field vector parallel to the major axis andtransmits the component of the s-polarized light with its electricalfield vector perpendicular to the major axis. Usually, the latergeometry has reduced efficiency and contrast compared to the onedescribed in FIG. 1 , however, for some applications, this geometry canalso be useful.

Another issue that should be considered is the required entranceaperture of the LOE. FIG. 14 illustrates the aperture of a conventionalLOE wherein the coupling-in element is a simple reflecting mirror 16 asdescribed above with reference to FIG. 2 . As illustrated, the inputaperture is determined by the marginal rays of the two extreme angles ofthe system's FOV. The rays 96 and 98 are the left and the right marginalrays of the left and the right angles of the FOV respectively. Theirintersections with the lower surface 26 of the LOE 20 determine theinput aperture S.sub.in1 of the LOE.

FIG. 15 illustrates the required input aperture for an LOE wherein thecoupling-in element is a polarizing beamsplitter 86 as described abovein reference to FIG. 12 . Since the two marginal rays 96 and 98 have tocross an additional thickness T of the LOE before reflecting back by thesurface 90 (FIG. 15 ), the optical path required before impinging on thecoupling-in element is longer than before. Hence, the required inputaperture S.sub.in2 is larger than the aperture S.sub.in1 of FIG. 14 .The difference between S.sub.in1 and S.sub.in2 depends on the variousparameters of the optical system. For example, in a system having ahorizontal FOV of 24 degrees, plate thickness of 2.5 mm and refractiveindex of 1.51, the difference between SS.sub.in1 and SS.sub.in2 is 1 mm.

FIG. 16 illustrates a method for significantly decreasing the requiredinput aperture. Instead of utilizing a flat reflecting surface aplano-convex lens 100 placed following the retardation plate 88 can beused. In order to couple collimated waves into the LOE, the waves 18that enter the LOE through the lower surface 26 should be divergent.These divergent waves are collimated by lens 100 and by the reflectionback of the wave from the reflective surface 102 of the lens 100. Thewaves are then trapped inside the LOE 20 in a similar manner to themethod described above with reference to FIG. 12 . The retardation plate88 could be cemented to, or laminated on, the front flat surface 104 ofthe lens 100. As illustrated in FIG. 17 , the input aperture S.sub.in3determined by the two marginal rays 96 and 98 is smaller than SS.sub.in1(FIG. 14 ). The extent to which this aperture is smaller than SS.sub.in1depends upon the optical power of the collimating lens 100. By utilizinga collimating lens instead of a flat reflecting surface, not only is amuch smaller input aperture achieved, but the entire optical module canbe much more compact than before as well.

FIG. 18 illustrates the entire optical layout of an exemplary systemutilizing the method described above. A folding prism 108 is exploitedto couple the light from a display source 106 into the LOE 20. The inputwaves 18 from the display source 106 are coupled into the prism 108 bythe first reflecting surface 110 and then coupled out by the secondreflecting surface 112, into the LOE 20 where they are collimated andtrapped.into the LOE in the same manner described above with referenceto FIG. 16 . The optical system illustrated in FIG. 18 could be muchmore compact than other conventional collimating modules. A possibledrawback of this layout is that the LOE, the collimating lens and thedisplay source are affixed together. There are cases however, where itis preferred to have the LOE 20 separated from the collimating module.For instance, in the optical system of eyeglasses, which is illustratedin FIG. 5 , the LOE 20 is integrated into the frame, while thecollimating module 50 is attached to the handle. It is thereforepreferred that mechanical tolerances between the LOE 20 and thecollimating module will be released as far as possible. One method toovercome this problem is to integrate the display source 106, thefolding prism 108, the retardation plate 88 and the collimating lens 100into a single mechanical body, leaving a space for the LOE 20 to beinserted.

A modified method is illustrated in FIG. 19 , wherein the collimatinglens is attached to the folding prism instead of the LOE 20. Asillustrated, the s-polarized input waves 18 from the display source 106are coupled into the prism 114 by the first reflecting surface 116.Following internal reflection from the lower surface 118 of the prism,the waves are reflected and coupled out off a polarizing beamsplitter120. The waves then pass through the quarter-wavelength retardationplate 122, are collimated by the lens 124 and the reflecting surface126, returned to pass again through the retardation plate 88 and enterthe prism 114 through the lower surface 118. The now p-polarized lightwaves, pass through the polarizing beamsplitter 120 and the uppersurface 128 of the prism and enter the LOE 20 through its lower surface26. The incoming waves are now trapped inside the LOE 20 in the samemanner illustrated in FIG. 12 . The collimating module 129 comprisingthe display source 106, the folding prism 114, the retardation plate 88and the collimating lens 124 can be easily integrated into a singlemechanical module which can be assembled independently of the LOE, withfairly relaxed mechanical tolerances.

In the embodiment illustrated in FIGS. 17 to 19 only a single sphericalconverging lens is utilized. For some optical schemes that may besufficient, however, for other systems having wide FOV and large inputapertures, better optical qualities may be required. One approach toimprove the optical properties of the system is to exploit eitheraspheric or even aspheric-diffractive lenses. Another approach is toutilize more than one imaging lens.

FIG. 20 illustrates an optical system utilizing a larger prism 130containing two embedded polarizing beamsplitters 132 and 134, aquarter-wavelength retardation plate 136 and two converging lenses 138and 140. As illustrated, the p-polarized input wave 18 passing throughthe first polarizing beamsplitter 132, is then reflected, partiallyconverged and changed to s-polarized light by the retardation plate 136and the first lens 138. The wave is then reflected by the firstpolarizing beamsplitter 132, the lower surface 142 of the prism 130 andthen by the second polarizing beamsplitter 134. It is then reflected,fully collimated and changed back to p-polarized light by theretardation plate 136 and the second lens 140. The wave then passesthrough the second polarizing beamsplitter 134 and enters into the LOE20 through the lower surface 26. The incoming wave is now trapped in theLOE 20 in the same manner as illustrated in FIG. 12 . The collimatingmodules illustrated in FIGS. 19 and 20 can be utilized not only for LOEsutilizing polarizing beamsplitters 86 as coupling-in elements, but alsofor conventional LOEs, wherein a simple reflecting minor 16 is utilizedas the couple-in element. Moreover, these collimating modules could alsobe exploited in other optical systems wherein the display source islinearly polarized (or alternatively, when brightness efficiency is nota critical issue) and when a compact collimating module is required. Acollimating optical module, similar to those illustrated in FIGS. 19 and20 having any required number of polarizing beamsplitters and imaginglenses could be utilized according to the required optical performanceand overall size of the optical system.

There are optical systems wherein the display source is unpolarized andwhere maximal possible efficiency is important. FIG. 21 illustrates anembodiment wherein another simple reflecting surface 144 is embeddedinside the LOE, parallel to the couple-in element 86. As illustrated,the s-polarized component of the incoming beam 18 is coupled into theLOE 146 by the surface 86, reflected by the surface 144, and is thenreflected and changed top-polarized light by the retardation plate 88and the reflecting surface 90. The reflected wave 150 is then coupledinto the LOE 20 by the reflecting surface 144. The p-polarized coupledlight 152 passes through the surface 86 and merges with the originalp-polarized component, which is trapped inside the LOE 20 in the samemanner illustrated in FIG. 20 .

For each instance where we have followed a particular polarized wavepath in the examples described above, the polarizations areinterchangeable. That is, on altering the orientation of the polarizingbeamsplitters, each mention of p-polarized light could be replaced bys-polarized light, and vice-versa.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

1. A method for fabricating an optical device, comprising: obtaining afirst light-waves transmitting substrate having a plurality of externalsurfaces including at least first and second external major surfaces anda third external surface non-parallel to the first and second externalmajor surfaces, the first light-waves transmitting substrate having acoupling-out optical arrangement for coupling light waves out of thefirst light-waves transmitting substrate; obtaining a second light-wavestransmitting substrate having a plurality of external surfaces includingat least first and second external major surfaces and a third externalsurface non-parallel to the first and second external major surfaces,the second light-waves transmitting substrate having a coupling-inoptical arrangement for coupling light waves into the second light-wavestransmitting substrate; and attaching the first and second light-wavetransmitting substrates together at the third external surface of thefirst and second light-wave transmitting substrates to form the opticaldevice such that the third external surface of the first and secondlight-wave transmitting substrates is a common surface to the first andsecond light-wave transmitting substrates.
 2. The method of claim 1,wherein the first and second external major surfaces of the firstlight-waves transmitting substrate are parallel to each other, andwherein the third external surface of the first light-waves transmittingsubstrate is normal to the first and second external major surfaces ofthe first light-waves transmitting substrate.
 3. The method of claim 1,wherein the first and second external major surfaces of the secondlight-waves transmitting substrate are parallel to each other, whereinthe third external surface of the second light-waves transmittingsubstrate is normal to the first and second external major surfaces ofthe second light-waves transmitting substrate.
 4. The method of claim 1,wherein the first and second external major surfaces of the first andsecond light-wave transmitting substrates are mutually parallel, andwherein the third external surface of the first and second light-wavetransmitting substrates are normal to the first and second externalmajor surfaces of the first and second light-wave transmittingsubstrates.
 5. The method of claim 1, wherein the coupling-out opticalarrangement includes at least one partially reflecting surface.
 6. Themethod of claim 1, wherein the coupling-in optical arrangement includesa reflecting surface non-parallel to the first and second external majorsurfaces of the second light-waves transmitting substrate.
 7. The methodof claim 1, wherein the coupling-in optical arrangement includes aprism.
 8. The method of claim 1, wherein the coupling in opticalarrangement includes a polarization sensitive beamsplitter.
 9. Themethod of claim 1, wherein the attaching the first and second light-wavetransmitting substrates together includes cementing the third externalsurfaces together.
 10. The method of claim 1, wherein the optical deviceincludes at least a first external major surface formed from the firstexternal major surface of the first and second light-waves transmittingsubstrate, and a second external major surface formed from the secondexternal major surface of the first and second light-waves transmittingsubstrate.
 11. The method of claim 10, further comprising: grinding orpolishing the first and second major external surfaces of the opticaldevice.
 12. The method of claim 1, further comprising: grinding orpolishing the first and second major external surfaces of the first andsecond light-waves transmitting substrates.
 13. The method of claim 1,wherein the obtaining the first light-waves transmitting substrateincludes: obtaining a plurality of substantially transparent plates, atleast one surface of at least one of the transparent plates being coatedwith a partially reflecting coating to form at least one partiallyreflecting surface, stacking the plates and attaching the stacked platestogether, and slicing the stacked plates along two parallel planes and athird plane non-parallel to the two parallel planes to form the firstlight-waves transmitting substrate, wherein the slicing along the twoparallel planes defines the first and second external major surfaces ofthe first light-waves transmitting substrate, and the slicing along thethird plane defines the third external surface of the first light-wavestransmitting substrate, and wherein the coupling-out optical arrangementincludes the at least one partially reflecting surface.
 14. The methodof claim 1, wherein the obtaining the second light-waves transmittingsubstrate includes: obtaining a first plate and a second plate, thesecond plate having a reflecting coating applied to at least one surfacethereof to form at least one reflecting surface on the second plate,stacking the first and second plates and attaching the stacked platestogether, and slicing the stacked plates along two parallel planes and athird plane non-parallel to the two parallel planes to form the secondlight-waves transmitting substrate, wherein the slicing along the twoparallel planes defines the first and second external major surfaces ofthe second light-waves transmitting substrate, and the slicing along thethird plane defines the third external surface of the second light-wavestransmitting substrate, and wherein the coupling-in optical arrangementincludes the at least one reflecting surface.
 15. A method forfabricating an optical device, comprising: obtaining a plurality ofsubstantially transparent plates, at least one surface of at least oneof the transparent plates being coated with a partially reflectingcoating to form at least one partially reflecting surface that coupleslight waves out of the optical device; stacking the plates and attachingthe stacked plates together; slicing the stacked plates along twoparallel planes and a third plane non-parallel to the two parallelplanes to form a first light-waves transmitting substrate, wherein theslicing along the two parallel planes defines parallel first and secondexternal major surfaces of the first light-waves transmitting substrateand the slicing along the third plane defines a third external surfaceof the first light-waves transmitting substrate non-parallel to thefirst and second external major surfaces of the first light-wavestransmitting substrate; obtaining a first plate and a second plate, thesecond plate having a reflecting coating applied to a surface thereof toform a reflecting surface on the second plate that couples light wavesinto the optical device; stacking and attaching together the first andsecond plates; slicing the stacked first and second plates along twoparallel planes and a third plane non-parallel to the two parallelplanes to form a second light-waves transmitting substrate, wherein theslicing the stacked first and second plates along the two parallelplanes defines parallel first and second external major surfaces of thesecond light-waves transmitting substrate, and wherein the slicing thestacked first and second plates along the third plane defines a thirdexternal surface of the second light-waves transmitting substratenon-parallel to the first and second external major surfaces of thefirst light-waves transmitting substrate; and attaching the first andsecond light-wave transmitting substrates together at the third externalsurface of the first and second light-wave transmitting substrates toform the optical device such that the third external surface of thefirst and second light-wave transmitting substrates is a common surfaceto the first and second light-wave transmitting substrates.
 16. Themethod of claim 15, wherein the attaching the first and secondlight-wave transmitting substrates together includes cementing the thirdexternal surfaces together.
 17. The method of claim 15, wherein theoptical device includes at least a first external major surface formedfrom the first external major surface of the first and secondlight-waves transmitting substrate and a second external major surfaceformed from the second external major surface of the first and secondlight-waves transmitting substrate.
 18. The method of claim 17, furthercomprising: grinding or polishing the first and second major externalsurfaces of the optical device.
 19. The method of claim 15, furthercomprising: grinding or polishing the first and second major externalsurfaces of the first and second light-waves transmitting substrates.20. The method of claim 15, further comprising: grinding or polishingthe first light-wave transmitting substrate prior to attaching the firstlight-waves transmitting substrate to the second light-wavestransmitting substrate.
 21. The method of claim 15, further comprising:grinding or polishing the second light-wave transmitting substrate priorto attaching the first light-waves transmitting substrate to the secondlight-waves transmitting substrate.